Estimating the cost of adding attic insulation

Q: I just got a bid for insulation of my 824-square-foot attic for $1,200. That seems pretty high.

Q: I just got a bid for insulation of my 824-square-foot attic for $1,200. That seems pretty high.

The contractor would blow in cellulose insulation. But you have said that expandable foam against the roof is better. Is the expandable foam you mentioned nontoxic? We have people in our home with upper-respiratory sensitivities.

A: For new construction, foam is an ideal choice, but in existing homes, insulating the underside of the roof's decking with foam is costly; sometimes two to three times that of conventional insulation. In addition, the existing attic insulation should be removed and a heating/cooling duct installed to control and condition the attic space. There is no out-gassing of the expanding foam insulation.

The price charged by a contractor will vary from city to city depending on wage rates, the complexity of the job and how much material will be required to gain the R-rating you need. The R-rating is the resistance to thermal transfer, and the higher the R-rating the better.

The insulation should never touch the underside of the roof's decking. A baffle should be installed between the rafters where the roof's decking is close to the attic's floor to provide airflow from the home's soffit vents and to prevent contact between the blown-in insulation and the decking.

If you choose foam insulation, there is no need for the baffles because all of the decking would be covered.

I did a quick check at one of the big-box stores, and found the following: a bag of loose-fill fiberglass insulation is selling for $31.79, and it would require 13 bags to cover 824 square feet of attic space to a depth of about 12 inches. The cost of blown-in cellulose would be similar, and it is installed using the same type of equipment as fiberglass.

The 13 bags of fiberglass would give you an estimated R-30 rating in addition to the insulation you already have.

Your estimate of $1,200 appears to be reasonable. Thirteen bags of insulation would cost you $413.27 plus tax, leaving around $700 for the contractor's labor and profit.

A common rule of thumb for estimating a small job is to take the costs of the material and double it for the labor and profit. Not a scientific method, but one often used by various contractors.

I would recommend you get at least three bids to compare, making sure each contractor is licensed (where required) and insured. Before hiring a contractor, contact your local Better Business Bureau for information on the contractor's business — and always get references from the contractor and contact his customers to see if they were satisfied with the work.

If you decide to do the work yourself, some home stores will loan you the equipment to install the insulation if you purchase a certain amount of the product. It takes at least two people to do the installation if you have communication devices such as handheld radios, or three people with one being the communicator. You have to know when to add insulation to the blower and when to stop.